Conventionally, integrated circuit chips are connected directly to a substrate preferably a "carrier" which in turn is connected to the circuit board. Plated through-holes extend from one surface of the carrier through to the other surface and electrically connect the top side, that is the circuitized side, of the carrier to the bottomside of the carrier. Pads, which have a center hole therethrough are positioned over plated vias or through-holes. On the underside of the carrier, are solder balls; the solder balls are attached to carrier pads. Solder balls connect the chip carrier to the circuit board.
There are several techniques for attaching the solder balls to the bottom of the carrier. In one technique, the carrier is positioned atop the solder balls and the carrier and solder balls are either placed in a furnace, or hot air is blown along the underside of the carrier, to reflow a portion of the solder balls. However, with both of these techniques, the carrier is also heated. Upon cooling, the carrier often warps or cracks due to differential rates of heating and cooling of the materials used in the carrier.
Another method for attaching solder balls to the underside of a carrier, known as "microsurface welding", involves contacting a pad on the topside of the carrier with two probes, one positive the other negative. Current is induced through the pad which heats the pad. The heat is conducted from the pad to the metalized walls of the plated through hole, along the plated walls of the through-hole, through to the pad on the underside of the carrier and to the solder ball, where it reflows a portion of the solder. The molten solder is drawn through the center hole of the pad on the underside of the carrier, up through plated through-hole and ideally through the center of the pad of the top side of the carrier. Unfortunately, with this technique, When the probes contact the pad, they often tilt the carrier in relation to the ball which leads to poor contact and poor reflow of solder. Also tips oxidize and/or deteriorate, thus requiring high contact force; to maintain such high contact force the tips' positions must continually be adjusted. There is also variability in the height of the solder column that is drawn into the through hole. When the solder column is too high, the solder column can penetrate into adjacent layers of the carrier thereby causing a short, or cause the dielectric film to be pushed from the stiffener leading to warpage. Where the column of solder is too short, the bond between the solder ball and the carrier is weak. On the average, a shear pull strength of only 240 to 250 g is obtained using this technique, due to lack of uniformity in solder column height.
Moreover, if an insufficient amount of solder from a solder ball reflows, this leads to a defective electrical and/or mechanical connection between the solder ball and the carrier. Once the carrier is attached to the circuit board, it is not only difficult to detect a poor connection, it also is extremely difficult to access the underside of the carrier to repair a defective solder ball connection. Thus, a single defective connection between a solder ball and the carrier could render defective the entire structure into which the carrier has been incorporated. As a result of a single defective connection, the entire structure might have to be discarded.
It would be desirable to have a technique for connecting solder balls to carrier pads that does not require heating the entire carrier, improves yield of carriers, improves average bond strength of the solder balls, and provides suitable solder column height.